Solved: infinite list

Haskell is a purely functional programming language known for its high-level features and abstraction. One remarkable area where Haskell’s power radiates is in working with infinite lists. With Haskell’s lazy evaluation, we can represent and manipulate infinite lists without running into issues of memory exhaustion unless we specifically ask to consume the list wholly. Imagine a list that goes on continually, like the numbers from 1 to infinity, such a list is an infinite list.

Infinite Lists in Haskell

In Haskell, there are numerous functions to deal with infinite lists. The most basic one is


. This function takes a value and produces an infinite list made of that value. For instance,

repeat 7

will yield an infinite list of sevens. Another useful function in this context is the


function. This function takes a function and a starting value. It applies the function to the starting value, then it applies the function to the result, then to the result’s result, and so on, generating an infinite list.

Infinite lists, though seemingly daunting, are easy to handle in Haskell, thanks to Haskell’s lazy evaluation model. This feature allows Haskell to evaluate an expression only when its value is necessary, hence providing an efficient way to work with infinite lists.

Coding with Infinite Lists

Let’s delve into some practical Haskell codes exploiting the concept of infinite lists. A common problem that we can solve with infinite lists is generating a list of all prime numbers.

The following code solves this problem elegantly:

primes = filterPrime [2..] 
  where filterPrime (p:xs) = 
          p : filterPrime [x | x <- xs, x `mod` p /= 0&#93;

In this code, the function &#91;code lang="Haskell"&#93;filterPrime&#91;/code&#93; takes the first number from the list (which is a prime) and concatenates it with the result of filtering out the multiples of that prime number from the rest of the list. The function &#91;code lang="Haskell"&#93;filterPrime&#91;/code&#93; then recursively calls itself to generate all prime numbers.

<b>With the above code, we not only solved our limitation but also illustrated the power and efficiency of Haskell's infinite lists.</b>

<h2>Understanding the Libraries</h2>

Haskell's standard library, GHC.Base, provides several functions that are crucial to the manipulation of infinite lists. These functions include [code lang="Haskell"]cycle



, and


, among others.

For instance,


function offers a simple way to create an infinite list. Meanwhile, the


function takes a finite list and replicates it infinitely.


, on the other hand, offers more flexibility as it allows us to generate an infinite list by repeatedly applying a function.

Understanding how to use these libraries and functions is fundamental to mastering infinite lists in Haskell. Thanks to these, creating and managing infinite lists become tasks that we can execute with ease and elegance.

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